WO94/20759 discloses an anti-stall tip treatment means in a gas turbine engine, in which an annular cavity is provided adjacent the blade tips of a compressor rotor. The cavity communicates with the gas flow path through the compressor through a series of slots defined between solid tip treatment bars extending across the mouth of the cavity.
Such tip treatments are applicable to both fans and compressors of gas turbine engines, and their purpose is to improve the blade stall characteristics or surge characteristics of the compressor.
In a gas turbine engine, blades of a rotating stage may become damaged or become detached from the rotating hub on which they are mounted. Damage of this type may be caused, for example, by impact or foreign object damage such as a bird strike. The blade, or fragment of a blade, which is shed can cause catastrophic damage to other parts of the engine. The consequential effects of blade shedding can be. particularly serious it the blade in question is in the compressor stage of the engine, and particularly near the front of the compressor stage, since such blades are the largest and heaviest in the engine.
If an entire blade is shed, it is preferable in many cases for the blade to break through the engine. casing to exit the engine, rather than to remain within the engine where it may cause catastrophic failure. Blade fragments, however, are best contained within the engine, but prevented from reaching later compressor or turbine stages.
To minimise the consequential effects of the shedding or disintegration of a blade, it is desirable for the engine to include means for containing blade fragments while enabling whole blades (or very large blade fragments) to break through the engine casing with the minimum of disturbance to the operation of the engine. It is known to provide woven material around the exterior of the engine casing in order to prevent blades or blade fragments from penetrating through the engine casing. Such measures may therefore minimise damage to the engine casing by absorbing the detached blade or fragment, but they do not adequately prevent the travel of the detached blade or fragment through the remaining stages of the engine,
Known tip treatment bars are solid and relatively robust and, in general, are as able as the adjacent parts of the casing to withstand impact from detached blades or blade fragments. They thus serve to keep detached blades and blade fragments within the engine, where they are liable to cause damage.
It is an object of the present invention to reduce the resistance of tip treatment bars to impact from detached blades or blade fragments.
It is a further object of the present invention to minimise the energy loss of a blade or blade fragment breaking through the.tip treatment bars.
It is a further object of the present invention to improve the damping characteristics of tip treatment bars.
According to the present invention there is provided a gas turbine engine casing having hollow tip treatment bars.
By making the tip treatment bars hollow, their resistance to impact from detached blades or blade fragments may be reduced. Consequently, entire blades can break through the tip treatment bars and the engine casing to exit the engine. Blade fragments may also break through the tip treatment bars, but they will lose some of their kinetic energy as they do so, and may then be retained within the cavity, or embedded in the engine casing, outside the gas flow path through the engine. Thus they may be prevented from causing damage to other parts of the engine. Smaller blade fragments may become lodged between the tip treatment bars, again with the result that those fragments are prevented from damaging further parts of the engine.
The tip treatment bars may be mounted between end supports connected to, or forming part of, the casing of the engine. The tip treatment bars may be integral with the end supports.
In a preferred embodiment, the tip treatment bars are thin-walled components, so as to minimise the energy loss of a blade or blade fragment breaking through them.
Tip treatment bars are vulnerable to high cycle fatigue failure, as a result of engine-induced vibration in them. By making the tip treatment bars hollow, in accordance with the present invention, additional measures may be taken to damp such induced vibrations, and so inhibit the initiation and propagation of fatigue cracking. For example, the hollow tip treatment bars may be filled, wholly or partially, with a damping material, Viscoelastic materials are suitable for this purpose and elastomers, such as silicone elastomers may be used.